1. Field of the Invention
The present invention relates to a switching regulator, and more particularly, to a switching regulator having a bypass switch which employs a circuit system that stably shifts from a bypass state to a normal state.
2. Description of the Related Art
A step-down switching regulator has been employed in every device since the step-down switching regulator is small in loss as compared with a series regulator. In recent years, particularly, the step-down switching regulator is frequently applied to mobile equipments such as cellular phones.
In particular, a power supply for driving an RF transmission amplifier of a cellular phone is greatly different in an electric power that is required by the RF transmission amplifier between a communication state and a non-communication state and also between a voice communication state and a data communication state. For that reason, when the power supply is brought to a state where the maximum electric power that is required by the RF transmission amplifier can be always supplied, the power supply capability when no maximum electric power is required is excessive with the result that the loss of the power supply increases. Because the cellular phone is driven by a battery, the increase in the loss of the power supply falls short of the needs of the market for a better cellular phone with a longer life battery.
Accordingly, it is necessary that the power supply for driving the RF transmission amplifier of the cellular phone can switch over the available electric power. As the power supply having a switchable electric power, there has been known a power supply using a chopper type step-down switching regulator (hereinafter, referred to as “step-down switching regulator”) (see Linear Technology Corp., LTC3408 data sheet).
FIG. 3 is a block diagram showing a conventional step-down switching regulator. The power supply of the step-down switching regulator can be switched over by making the output voltage Vout variable. When the required electric energy is larger, the output voltage Vout is increased, and when the required electric energy is smaller, the output voltage Vout is decreased.
The conventional step-down switching regulator is equipped with a bypass transistor 104 that is a bypass switch which short-circuits a series resistor consisting of a MOS transistor 100 and an inductor 102. Also, the conventional step-down switching regulator controls a voltage of a reference voltage circuit 109 by the aid of a change-over comparator 108, to thereby render the bypass transistor 104 conductive when a large electric energy is required. As a result, Vin and Vout are short-circuited to make the output voltage Vout variable.
In this situation, an on-resistance of the bypass transistor 104 is made remarkably smaller than an on-resistance of the MOS transistor 100 to suppress the power loss.
However, in the conventional step-down switching regulator shown in FIG. 3, in a case where a change rate of the reference voltage circuit 109 is high, there arises such a problem that an overshoot or an undershoot occurs in an output voltage of the Vout terminal as shown in FIG. 4, and the output voltage is not stabilized.